Thermal management of the auxiliary power unit (APU) compartment for a high-powered aircraft presents a challenge. A suitable cooling system for such an application must insure that the APU oil, that is, the oil used to lubricate and carry heat away from the APU and the generators that load the APU, as well as the air in the compartment that houses the APU is properly cooled.
Typically, a cooling system generates and directs an air stream through fins of air-cooled APU oil cooler to cool the APU oil. This air stream may comprise ambient air, compartment air, or a combination thereof. An active means, such as a powered fan, or a passive means, such as a well-known eductor, otherwise known as a jet pump or ejector, may generate and direct the air stream through the APU oil cooler. A passive system is advantageous from the standpoint of weight reduction and reliability.
Compartment air is preferably replenished from ambient to aid in cooling the APU compartment. In a passive cooling system, the compartment air typically combines with the exhaust gas stream of the APU. The high velocity exhaust gas of the APU provides an eduction effect that pumps the low velocity air out of the APU compartment. Circulation of the compartment air in this way draws cool ambient air into the compartment through a suitable aperture in the compartment, and this serves to cool the compartment.
It is advantageous to pass the circulated compartment air through the fins of the oil cooler to more effectively cool the APU oil. Therefore, appropriate ductwork may direct the stream of evacuated compartment air through the oil cooler as it is pumped from the APU compartment.
Current passive cooling systems are bulky and complex, and their ability to distribute and ventilate compartment air is limited. Furthermore, they do not have any inherent ability to protect the oil cooler from flames that may emanate from the APU or pooled fuel in the compartment.